Evaluating the effectiveness of the phosphorus sorption index for estimating maximum phosphorus sorption capacity

The single-point P sorption index (PSI), which is defined as the ratio of sorbed P (S) to the log P concentration in soil solution following a single P addition, is often used to estimate maximum soil P sorption capacity (S_max). Although studies have found good correlations between PSI and S_max as determined from fitting the Langmuir model to complete sorption isotherm data, a thorough analysis of the role of added P concentration on this relationship is needed. Our first objective was to investigate the effect of added P concentration on the correlation between PSI and S_max as determined by the Langmuir equation. Our second objective was to determine if S was better than PSI for predicting S_max. Using numerical simulations, we tested the correlation between S_max and PSI for added P concentrations of 75, 100, 150, and 200 mg P L⁻¹. Results of the simulations show that the strength of the correlation between S_max and PSI increases with increasing P concentration. Our results also show that PSI was a better predictor of S_max than S for added concentrations of 75 and 100 mg P L⁻¹, whereas at the higher rates S was a slightly better predictor of S_max and gave a direct estimate of S_max rather than the relative estimate obtained from PSI. Results from P sorption data measured on soils from Maryland and Sweden were consistent with our results from the numerical simulations. Our findings highlight important limitations of using PSI for estimating S_max.